With heat flux dissipation levels from small form factor devices skyrocketing, conventional cooling methods like air flow over extended surfaces etc. have been unable to keep pace with the cooling demands. Cooling by single phase flow through micro-channels have gained tremendous popularity as a promising alternative and is being commercialized rapidly. However, significant temperature variations across the heat sink persist since the heat transfer performance deteriorates in the flow direction in micro-channels as the boundary layers thicken and the coolant heats up by sensible heat gain. Hence, for very high heat flux dissipation from narrow spaces, flow boiling through micro-channels is evolving as a preferred cooling solution.
Two-phase flow instabilities may arise when boiling occurs in conventional size channels and more so in a parallel array of multiple micro/mini-channels. These undesired effects must be controlled or mitigated because they can induce mechanical vibrations in the system, degrade the heat transfer performances (premature dryout, CHF limitation) etc. Two-phase flow instability is a complex topic because several effects may occur simultaneously and play a role in a coupled way.
The classical theories developed for conventional size channels can be used to a limited extent for interpreting the instabilities phenomena observed in micro/mini-channels. In fact, when dealing with phase-change phenomena the basic mechanisms such as nucleation, coalescence, fragmentation, and interfacial instabilities still exist. Nevertheless, when micro/mini-channels are involved, some differences exist. For instance, in a small size channel the vapor growth phase is limited in the radial direction because of the hydraulic diameter. Only the axial direction allows vapor growth when boiling occurs. As a result there are important differences observed in the physical processes when compared to conventional macro-scale systems